Chapter 8 certainly appears to have been based on Alma’s personal record, so this header does not indicate a new source, but clarifies where the quoted material comes from. Although other chapter headers appear to cover only the chapter they head, in this case Mormon appears to give us a header that covers multiple chapters. Mormon notes that “they are cast into prison and delivered by the miraculous power of God.”
At the end of Chapter 9, in verses 32 and 33, Mormon tells us that an attempt was made to cast Alma and Amulek into prison, but that the people were unable to do so. That doesn’t fit the header’s description of being in prison and miraculously delivered. That does happen, but not until Chapter 14. Although Orson Pratt divided some of the intervening chapters, there were, nevertheless, two whole chapters in the 1830 edition, before the story occurs, that fits the information in this header.